As compared to single ended converter circuits, full bridge converter circuits have a number of attractive features for high power dc-dc converters, such as balanced use of the output rectifiers and reduced output filter requirements. Although the details of operation may differ for such varieties as square wave or resonant transition full bridge circuits, all well-known bridge or symmetrical drive circuits apply an alternating positive and negative voltage to the primary winding of the power transformer. In the transition period between the application of positive and negative voltage, there is an intervening zero voltage or freewheel time interval as may be required to accomplish regulation.
For high output voltage levels, such as might be used for motor drives or relays, the output from the secondary winding of the transformer is typically rectified using four rectifiers in a full bridge rectifier arrangement. However, for applications requiring low voltages and high currents, such as high performance logic circuits, the two rectifier forward voltage drops encountered in a full bridge rectifier will cause an unacceptable power loss. For these low voltage, high current applications, a center tapped secondary winding is typically used in conjunction with two rectifiers to obtain full wave rectification. This arrangement eliminates the unacceptable power loss due to the full bridge configuration since only one rectifier forward voltage drop is encountered.
In very high current applications, conventional wire cannot be employed as conductors in the secondary structure precisely because of the high current passing through the conductor. To overcome this limitation, bus bars or copper plates or disks are used to accommodate the high current in these types of transformers. A significant difficulty in using either a bus bar or a copper plate or disk for the secondary structure is that it is mechanically awkward to make the required connections (rectifier and center tap) to the secondary structure. FIG. 1 illustrates an exploded view of a prior art bridge transformer and one manner of making the connections to the secondary structure. Elements 10 and 20 are the two halves of a conventional E--E shaped magnetic core. Encircling center post 30 of the core are the primary winding, 40, and the secondary winding, 50. As illustrated in FIG. 1, the secondary structure, 50, is constructed from a solid plate or sheet of conducting material. The secondary contains a center tap piece, 60, and contacts, 70 and 80, for making connections to the anode sides of the two rectifying diodes 90 and 100. The cathode sides of the rectifiers are shown commonly connected to the external circuit (shown as a filter-load comprising a capacitor and a load resistor R in FIG. 1). The other connection to the external circuit is shown from the center tap 60. Typically the rectifiers, 90 and 100, and the connections thereto, are external to the transformer structure. For an increase in the length of the connection to the external rectifiers, there is an increase in the inductance of the transformer circuit and thereby a direct degradation in the electrical performance of the transformer. Both the rectifier connections to the secondary and the rectifier connections to each other should be as short as possible to maintain low inductance.
In high current transformer applications, the transformer will typically provide a large voltage step-down ratio and the different structural composition of the primary and secondary windings required to achieve this step-down will provide additional fabrication and assembly challenges. In such a transformer, the primary winding will contain many turns (typically on the order of 15 to 50) of relatively flexible conductor, such as wire, flat wire, or braid. In FIG. 1, the primary winding, 40, is shown as a planar, multi-turn coil of wire conductor. The primary will carry modest currents of 5 to 20 amps, peak alternating current (AC) with no direct current (DC) component. The primary winding must be insulated to withstand hundreds of volts to function, and perhaps thousands of volts to meet safety requirements. In comparison to the primary, the secondary structure contains relatively massive and inflexible parts, such as the copper contact plate, 50, shown in FIG. 1. The secondary structure will carry hundreds of amps with both AC and DC components. Fifty volts functional insulation is adequate for the secondary.
The vastly different character of the primary and secondary structures of such a transformer are likely to require different fabrication, assembly, and mounting techniques. Yet, in order to provide tight magnetic coupling between the primary and secondary structures, which is required by some high performance transformers, it is necessary that the two structures be in close physical proximity. In FIG. 1, both the primary and secondary windings are mounted on core post 30 with the secondary contact plate, 50, lying directly beneath the primary coil 40. The primary coil, 40, could be wound in a cylindrical fashion, up and down the core post to create more turns on the post, but the primary is preferably wound in a planar fashion. The planar winding is more desirable in order to effect a low profile (height dimension) for the overall transformer and to enhance magnetic coupling to the secondary structure.
It is therefore one object of this invention to improve the electrical performance of a low profile bridge transformer.
It is also an object of this invention to provide tight magnetic coupling between the primary and secondary structures of such a transformer.
It is another object of this invention to facilitate fabrication assembly and mounting of a transformer structure.